Tell your friend to sale his 300 and buy a 25-06. I've had more deer over the years drop in their tracks with the 25-06 shooting a 115 gr. Ballistic Tip. For some reason most of the deer I've taken with the 06 fall on the spot. I also have taken many with a 280 Rem, 257 Roberts, 270 Win, 300 Win Mag, 243 Win, & 7MM Rem Mag. I can only think of one dropping with the 270 & probably 3-4 with the 280. The results are usually 20-40 yards before dying. Tell your friend he will be able to see the deer fall through the scope on the 25-06 and not get tagged in the face like with the 300 WSM. Retire the 300 until he is elk hunting.
Bullet construction and point of aim mean more then any particular cartridge. I have shot a ton of deer with a Rem 7mm and 270 WIn,300 WIn mag. Few ran more then 40 yards tops most just fell over or were launched backwards using standard jacketed bullets . I killed 4 deer last year with a 7WSM(150 powerpoint) and all but one dropped at the shot. One buck actually went 150 yards with both lungs blown out, that would be the longest tracking job with any 7mm I have used to date. I have not had a 280 drop anything in its tracks and the same for a 300 wthby, they make their 50 yard death run and fall over..Nosler partitions were used in both rifles . Deer were absolutely hammered by a 300 wsm shooting 180 grain ballistic tips. In retrospect I probably should have concentrated more on shot placement with my 280 and 300 wthby and used a lighter constructed bullet but they ran but I didn't lose them. A 260 remington is a fine rifle it just got pigeonholed by the gunwriters as usual and its effectiveness will be lost on a few of us that don't own them. When my son gets old enough to hunt(Next Year) i will look at them as well as 7-08 and maybe a 25-06 or 25wssm since they seem to be compact and lightweight.